Add endpoint for cancelling a build

- Adds a new BuildStatus: CANCELLED
- Adds an endpoint for changing the state of a build.  Currently only
the status can be changed and it can only be changed to CANCELLED.
- When a build is cancelled, we deplete any unstarted subjobs, then
call teardown on each slave.
- In the teardown call on a slave, wait for all executors to be idle
before posting idle status to the master.  This guards against a race
condition where a new build is submitted to the slave before all
executors are marked idle
- Client will cancel a build it has started if it is Ctrl-C'ed
I think you're right, we should add a "git fsck" check.  If it fails then we delete and reclone the repo.  I'll put that in another pull request